The most rapidly rising HIV infection rates in the world are in Eastern Europe. Bulgaria and Hungary are countries at still-early points in which are likely to become sharp HIV growth curves. Prevention efforts undertaken now can avert explosive HIV/AIDS epidemics throughout Eastern Europe being driven by social, behavioral, cultural, and economic changes since the collapse of the Soviet bloc. In both Bulgaria and Hungary, HIV epidemiology involves primarily sexual transmission. Two populations at especially high risk are young men who have sex with men (YMSM) and Roma (Gypsy) men and women, the largest, most marginalized and most vulnerable ethnic minority group in Eastern Europe. Both YMSM and Roma in Eastern Europe are likely to be distrustful of official pronouncements about AIDS but more trusting of personally known sources. A potentially powerful level of HIV prevention is intervening "inside" of intact high-risk social networks by training network leaders to function as behavior change agents to their own network members. This study will begin with a period of in-depth ethnographic formative research in Roma and YMSM communities in Budapest and Sofia to examine characteristics of high-risk social networks and to pilot test network recruitment, assessment, venue identification, and intervention methods. A main intervention trial will then be undertaken which recruits and enrolls 40 YMSM and 40 Roma high-risk social networks (20 networks per subgroup per country, total n=640), randomized in equal numbers to a control condition (risk reduction counseling for all members) or an intervention condition. Sociometric measures will identify the social influence leaders of each intervention network, and leaders will attend a 9 session program that trains and guides them in delivering theory-based risk reduction messages and peer counseling to personal network members. Their messages will target network members? safer sex knowledge, attitudes, intentions, perceived norms, and self-efficacy, all factors found in our pilot work to predict risk in the same populations. At baseline and at 3-and 12-month follow-up, participants will be assessed to determine their HIV risk and offered STD testing. We hypothesize reductions in sexual risk behavior, higher condom use, and lower STD incidence among members of intervention condition social networks. The trial will then replicate the intervention with former control networks and follow the intervention condition networks for an additional 12 months. This study will evaluate a model for undertaking HIV prevention within high-risk social networks, a promising approach understudied in the field.